A feed-forward equalizer circuit is a transversal filter that may be employed in multi-gigabit/s serial link transceivers to compensate for the frequency dependent loss of communication channels. Since these channels typically have a low-pass transfer function, an FFE is programmed to have high-pass characteristics to effectively compensate for the channel transfer function and reduce intersymbol interference introduced by the channel. In principle, an FFE can be implemented in either a transmitter or a receiver.
Traditionally, FFE realization in the transmitter has been preferred due to its ease of implementation and lower power and area costs. However, placing an FFE in the transmitter has important disadvantages. Since the quality of the received signal is only known in the receiver, automatic adaptation of the FFE coefficients for optimum link performance requires a back channel from receiver to transmitter, something which is often unavailable (especially when the transmitter and receiver are sourced from different vendors). Since the peak voltage swing of the transmitter is limited by available supply voltages, transmit-side FFE compensates for high-frequency loss in the channel not by increasing the amplitudes of high-frequency signals but by decreasing the amplitudes of low-frequency signals. This results in a smaller received signal, which is more vulnerable to noise sources such as crosstalk.
One way to avoid the disadvantages of transmit-side FFE is to implement the equalizer in the receiver. To avoid the difficulties of implementing an actual receive-side FFE, peaking amplifiers are commonly employed in serial link receivers. However, peaking amplifiers do not provide enough flexibility in the placement of their poles and zeroes, making it difficult to accurately match the equalizer to the channel characteristics. Moreover, peaking amplifiers are not compatible with well-known equalizer adaptation algorithms (e.g. Least Mean Squares or LMS). Thus, there is a strong motivation to develop circuit techniques that overcome the practical difficulties of implementing receive-side FFE systems.